1. Field
The disclosed concept pertains generally to electrical systems that employ AC transformers, such as, without limitation, a variable frequency drive employing an isolation transformer or an electrical distribution system employing a distribution transformer, and, more particularly, to a system and method for magnetizing the transformer prior to energizing the electrical system from the main AC source.
2. Background Information
A voltage source inverter is often used to power a motor, such as an induction or synchronous motor, or a generator, with a suitable medium voltage. One example of a voltage source inverter is a variable frequency drive (VFD), which controls the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. VFDs are also known as adjustable frequency drives (AFDs), variable speed drives (VSDs), AC drives, microdrives or inverter drives. Since the voltage is varied along with the frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.
Typically, a VFD first converts an AC input power to a DC intermediate power. The DC intermediate power is then converted to a quasi-sinusoidal AC power for driving the motor. Thus, the main components of a typical VFD include a number of input isolation transformers coupled to the source of AC power, a converter, such as a number of rectifier bridge assemblies, for converting the AC source power into the DC intermediate power, a direct current (DC) bus and associated DC bus capacitors for storing the DC intermediate power, and an inverter for converting the stored DC intermediate power into a variable voltage, variable frequency AC voltage for driving the motor.
One problem encountered by VFDs is caused by the fact that, when a transformer is first energized, a transient current up to 10 to 15 times larger than the rated transformer current can flow for several cycles. This transient current is known as inrush current. The magnitude of the inrush current may cause fuses to open, breakers or contactors to open, and protection relays to “false trip”. For large drives, this problem is significant in that the power system must be sized to provide the transient in-rush currents. Eliminating the inrush is of significant advantage as it increases reliability and/or reduces system cost.
A second problem encountered by Voltage Source Inverters is charging the large capacitors during initial energization to prevent damage to rectifier, fuses and associated circuitry.
The above-described problem of inrush current is not limited to VFDs. Rather, inrush current is a problem for any electrical system that utilizes a (large) transformer, such as, without limitation, an electrical distribution system that employs a distribution transformer or any industrial equipment that employs a drive having an input transformer. There also needs to be a method of pre charging the capacitors.
There is thus a need for a system and method for effectively reducing and/or eliminating inrush current in electrical systems that utilize input transformers.